Semiconductor lasers are compact and have high output, and allow for highly efficient electro-optical conversion, with the conversion efficiency exceeding 40%. A semiconductor laser has an optical waveguide structure, and high efficiency is achieved by utilizing optical trapping within an optical waveguide. However, this trapping of light limits how much the output of a semiconductor laser can be increased. This is because when the power density in an optical waveguide rises, the power density near the exit portion also rises, which can lead to end face breakdown, and reliability also suffers. An effective way to solve this problem is to reduce the power density of the light of a semiconductor laser. Thus, the power density of light is reduced, and output is increased, by increasing the cross sectional area of the optical waveguide in which light is trapped. A semiconductor laser such as this is called a wide stripe semiconductor laser (hereinafter referred to as a semiconductor laser), and has high output characteristics ranging from a few hundred milliwatts to a few watts.
Meanwhile, it is difficult to keep the light that propagates through the optical waveguide in a semiconductor laser in single mode, and the electric field distribution of the light becomes non-uniform because of the presence of a number of multimodes. Also, with a semiconductor laser, unlike the single-mode semiconductor lasers ordinarily used in optical communications or optical disks, there is a large decrease in convergence characteristics. Also, because a plurality of modes are present, oscillation at a single wavelength is difficult. Consequently, coherence deteriorates both spatially and temporally, and application to single-mode fibers or optical waveguide devices has been difficult.
Providing optical feedback to a semiconductor laser has been proposed as a way to solve these problems. The waveguide mode of a semiconductor laser can be controlled by external optical feedback. For instance, it has been indicated that the oscillation wavelength of a semiconductor laser can be fixed by feeding back the light emitted from a semiconductor laser in a resonator of the semiconductor laser after subjecting the light to wavelength selection with a narrow-band wavelength selecting filter or fiber grating (see Non-Patent Document 1, for example). Control of the lateral mode of a semiconductor laser is also possible, and a method has been proposed in which a semiconductor laser is oscillated in single mode by using a nonlinear mirror to return light from the outside (see Non-Patent Document 2, for example).
Non-Patent Document 1: Optics Letters, Vol. 22, No. 16, pp. 1250-1252 (1997)
Non-Patent Document 2: Optics Letters, Vol. 23, No. 11, pp. 825-827 (1998)